1. Field of the Invention
The present invention relates to a motor control device for operating a motor having a stator with a stator coil concentrically arranged on the outer side of a rotor having a permanent magnet, the motor control device including magnet temperature estimating means for estimating a temperature of the permanent magnet, and control means for controlling the motor based on the estimated magnet temperature. The present invention also relates to a control method and a control program of such motors.
2. Description of the Related Art
Recently, there has been increasing interest in hybrid vehicles, electric vehicles, and vehicles powered by fuel cells, and various attempts have been made to improve the driving performance of such vehicles to go beyond conventional vehicles driven by internal combustion engines. For example, in hybrid vehicles, attempts to increase the energy efficiency of vehicles to improve the driving performance are under way by using higher voltages, modifying the motor driving system, and so on.
Use of the motor in higher output state for an extended period of time, however, may cause the stator and rotor to exceed its upper limit temperature, which leads to demagnetization of the permanent magnet of the rotor. This results in a decreased torque of the motor. To avoid this, it is important to control the motor to not exceed the upper limit temperature, while improving the cooling ability of the motor, so as to restrict the rise in internal temperature of the motor.
For precise control of the motor temperature, the temperature of the rotor and stator should be measured. Typically, the stator is fixedly attached to the motor casing, and the temperature of the stator can be measured easily by attaching a thermocouple to the stator. However, because the rotor continues to rotate inside the stator, a slip ring or a rotary connector is needed on the shaft of the rotor to take out signals from the thermocouple attached to the stator. As a result, the cost of the motor might increase and the structure of the motor becomes more complicated.
Japanese Patent Laid-Open Publication No. 2005-73333 discloses a technique to estimate the temperature of a magnetic sensor, a bearing, a rotor magnet, and so on, by previously measuring and storing a temperature distribution of the motor, detecting a temperature of the stator coil using a plurality of thermistors, and comparing the detected temperature values against the stored temperature distribution of the motor.
In addition, Japanese Patent Laid-Open Publication No. 2000-23421 discloses another technique to estimate the temperature of the rotor magnet, in which the oil cooling system on the stator side is separated from that on the rotor side, the flow rate of the cooling oil circulating through the oil cooling system of the rotor is measured, and the temperature of the incoming cooling oil and the outgoing cooling oil, respectively, before and after the rotor is cooled by the rotor cooling system is also measured to estimate the temperature of the rotor magnet based on the difference of temperature of the cooling oil.